Data is often used to show geographical information. Being able to use measures of data, make calculations and explore relationships is an essential geographical skill.

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Calculating percentage increase is an important skill for geographers to have. When geographers collect data over a period of time, the results may increase. Calculating a percentage increase allows a geographer to see how much their data has changed. For example, it may be useful to find out how much the width of a river channel increases as you travel downstream.

Method:

- Work out the difference between the two numbers being compared.
- Divide the increase by the original number and multiply the answer by 100.

In summary: **percentage increase = increase ÷ original number × 100**

For example: the number of robins in a woodland area are counted over two different months. In December **15 robins** were counted. In January **23 robins** were counted. What is the percentage increase of robins in the woodland?

- The difference between the two numbers is
**8**. **8 ÷ 15 × 100 = 53.3**- The percentage increase of robins found in the woodland is:
**53.3%**

Calculating percentage decrease is also a useful skill to have. For example it may be useful to find out how much the load particle size decreases in a river as you travel downstream.

Method:

- Work out the difference between the two numbers being compared.
- Divide the decrease by the original number and multiply the answer by 100.

In summary: **percentage decrease = decrease ÷ original number × 100**

For example: the number of robins in woodland in February and March are counted. In February **22 robins** were counted. In March **12 robins** were counted. What is the percentage decrease of robins in the woodland?

- The difference between the two numbers is
**10**. **10 ÷ 22 × 100 = 45.4**- The percentage decrease of robins found in the woodland is:
**45.4%**

Percentiles and quartiles are both ways of dividing data into smaller parts. Whereas quartiles divide a set of data into 4 equal parts, percentiles divide the set of data into 100 equal parts.

Percentiles are commonly used to plot the growth of babies. For example, a midwife weighs baby Anna and she is in the 90th percentile. This means that if there were 100 babies (of the same age), 90% of them would weigh the same, or less than baby Anna and 10% would weigh more.